The technology of encoding information on various articles with bar codes is well known. Traditional bar code systems rely on the differences in reflection of the reading light from the black (light-absorbing) bars and the white (light-reflecting) spaces of the bar code. A typical laser bar code reader scans the beam from a helium-neon laser across the bar code. Photodetectors monitor the beam's reflectance from the bars and spaces, and the resultant electronic signals are processed and decoded.
Bar codes on mail can be used in a system for automatic sorting of mail in the postal system..sup.2 For this application, black bars would be difficult to read over dark backgrounds and could obscure underlying printed material. For this reason luminescent, substantially transparent bar codes are preferred. Luminescent bar codes may include both fluorescent and phosphorescent materials that can be formulated into relatively clear inks that do not obscure the underlying printed material on mail pieces.
Bar code readers require sufficient contrast between bars and spaces to permit accurate differentiation. With fluorescent bars, a major problem has been the background fluorescence of the paper in the spaces of the bar code. Various ways of compensating for differences in background fluorescence of the spaces have been devised by use of optical filters and electronic circuitry (U.S. Pat. No. 3,207.910 Hirschfeld) and development of fluorescent inks that can be activated by ultraviolet light to fluoresce at longer wavelengths (e.g. 580 nm peak) where the background fluorescence of the paper is less (U.S. Pat. No. 4,186,020, Wachtel). While methods are known to deal with the background fluorescence of the paper, which may appreciably affect the signal from spaces in the bar code, they do not address the problem of the background under the bars of the bar code.
One type of system uses fluorescent ink in bar codes that fluoresce orange-red at a wavelength of 600 nm when illuminated with ultraviolet light with a wavelength of 365 nm .sup.3. It was observed that the fluorescent signal on white envelopes was much larger than on dark envelopes. Thus, the problem was presumed to be partly related to the use of fluorescent substances ("whiteners") in white paper, resulting in a substantial dynamic range of signal. Thus, the detection circuit had to cope with white and dark paper. Advanced self-adapting thresholding techniques were said to have solved this problem of dealing with different envelope materials. However, the problem of background differences within the code area was noted by the same author to cause damage to the signal and errors in decoding. Based on the erroneous assumption that "whiteners" in the paper were the problem, a change from ultraviolet to green light for exciting fluorescence did not solve the problem.
Another system uses phosphorescent ink in bar codes to avoid the problems of fluorescent materials in the paper or printing inks..sup.4 However, differences in phosphorescent signals over white and manilla envelopes were still encountered. To minimize the problem, a scanning system was used ahead of the jet printer to adjust the number of ink droplets per bar in accordance with the color density of the envelope. However, this system would not be able to cope with variations in background within the code area.
Still another type of fluorescent system is based on dyes that fluoresce in the far-red and near-infrared region of the spectrum..sup.2 In this region, postal materials have little or no background fluorescence and the signal is obtained primarily from the fluorescence of the bar code. A further advantage of this system is that the helium-neon laser of the conventional bar code scanner can be used as the excitation light for the fluorescent dye. The principal modification to the reader is replacement of the optical filter on the conventional system which is designed to pass the reflected red light of scanner while blocking ambient light. The replacement is an optical filter that blocks the reflected red light of the helium-neon laser but passes the fluorescent signal of longer wavelength. Since the fluorescent bars produce the stronger electronic signal (in contrast to the conventional black and white system where the spaces produce the stronger electronic signal), the voltage signal must be inverted for feed to the conventional decoder circuitry.
With this red-stimulated fluorescent system, as with the ultraviolet-stimulated fluorescent system and the phosphorescent system, there is the common problem of the effect of the background on the dynamic range of the luminescent signal. While methods are known that might compensate for differences in envelope material, they would not solve the problem of variations in the background within the code area. Specifically, a relatively clear bar code of luminescent material printed over dark typewritten material presents the problem of localized variations in signal intensity that can cause errors in decoding. Variation in background can occur within the width of a single bar or space of the code.
The exact details of the mechanism that causes the quantitative increase in luminescence over lighter backgrounds is not known and probably differs slightly for fluorescent and phosphorescent materials. However, a qualitative explanation of the effects observed can be provided for the example of a bar code ink using a dye that is fluorescent in the far-red and near infrared region, where the fluorescence of the envelope or printed material can be neglected.
First consider the example of a bar formed of dispersed fluorescent dye particles in a relatively clear ink on an ideally non-reflective surface. When the excitation light impinges on the dye particle, it fluoresces in all directions. However, only that light fluorescing in the direction of the collector contributes to the signal strength. Second consider the same bar on a reflective surface. A portion of the light from the fluorescent particle is transmitted through the clear ink and is reflected from the reflective surface back to the collector and adds to the fluorescent signal. Thus, the fluorescent light received by the collector comprises two components: direct fluorescence and reflected fluorescence. The direct fluorescence is primarily a function of the dye concentration in the ink whereas the reflected fluorescence is a function of reflectance of the reflective background surface that the fluorescent ink covers.
It is known that the relative contribution of the background can be minimized with thicker bar codes. However, for noncontact printing of bar codes with jet printers there are practical limitations on film thickness. Thus, the thin fluorescent ink films of a jet printer accentuate the problem of variable background reflectance. It is known that if the ink is made less clear, the contribution of the reflected fluorescent component can be minimized. In the extreme case of an opaque ink the reflected fluorescent component can be made negligible and only the direct light of surface fluorescent particles would provide the signal, which would be independent of the background. However, any significant increase of the opacity of the bar code ink would be counter to the objective of not obscuring the underlying printed material.
For successful decoding with luminescent bar codes, it is necessary that the processed signal from the code delivered to the decoder have practically constant amplitude, independent of any variations in the collected luminescent signal due to localized variations in background reflectance. Thus, means are needed for automatically compensating for the total collected luminescent signal for the reflected component of the luminescent light according to the localized background surface reflectance in the area of the scanning beam. The present invention provides such means.
It is well known to use optical filters to separate light of different frequencies. For example, in conventional bar code readers, a red filter is used to block frequencies of ambient light and pass only the frequency of the red light of the helium-neon laser to measure the reflected red light from black on white bar codes. To adapt this conventional system to reading red-stimulated infrared fluorescence, the conventional ambient-light filter is replaced by an optical filter that blocks the red light of the helium-neon laser and passes the fluorescent light in the far-red and near-infrared range of the spectrum..sup.2 The amount of red excitation light to be blocked by the filter depends on the amount reflected by the background on which the fluorescent bar code is printed. It is greatest for a reflective white surface and least for a dark light-absorbent surface such as black printing ink.
If, in addition to blocking the reflected red excitation light in order to selectively measure fluorescent light, separate means are provided to simultaneously measure the amount of reflected excitation light, a separate electrical or optical signal can be obtained that is related to the reflectance of the surface underlying the fluorescent bar code; and this signal can be used to simultaneously compensate the amplitude of the fluorescent signal by appropriate electronic or optical circuitry to provide a relatively constant amplitude of the identification signal fed to the decoder. The above combination of dual light collection and detection means with background compensating electronic or optical circuitry means comprise a significant feature of the present invention, which significantly improves the ability of the system to successfully decode fluorescent bar codes printed with transparent ink over backgrounds of variable reflectance.
This invention is described in further complete detail in a technical publication 1 by two of the inventors and another coauthor, which is incorporated herein by reference as fully as if it were presented in complete text. Also similarly incorporated by reference are the other documents appended to the specification in the file of the parent application for this patent, and hereinafter identified.